Structuring of silicon by plasma etching methods is known. In plasma etching methods, chemically reactive etchant species and electrically charged particles, i.e., ions, are produced in a reactor with the help of an electric discharge in a reactive gas mixture. Positively charged ions generated in this way are accelerated toward the substrate by an electric bias applied to the silicon substrate, so they strike the substrate surface approximately perpendicularly, promoting the chemical reaction of the reactive etchant species with silicon on the substrate to be etched. A distinction is made between ion-induced anisotropic plasma etching, wherein the etching action of the etchant species is linked to a high-energy current of ions toward the substrate surface, and spontaneous isotropic plasma etching, wherein the etchant species require very little or no ion support. Ion-induced plasma etching, which is preferably performed with less reactive halogens such as chlorine or bromine when silicon is the material to be structured, has comparatively low erosion rates and is extremely susceptible to moisture and plasma impurities. However, spontaneous plasma etching, which is performed mainly with fluorine gases when silicon is the material to be structured, has high etching rates and, furthermore, highenergy ions are not needed. Therefore, in a search for suitable mask materials, only chemical factors regarding the selectivity between mask and substance to be etched need be taken into account, thus greatly expanding the selection options. In many cases, when materials other than silicon are to be structured, masking is possible, if at all, only with spontaneously reactive plasma chemistry. A very high mask selectivity can be achieved with this approach, because very little or no ion-induced or physical mask erosion occurs at low ion energies, and purely chemical reactions are extremely material-selective. Likewise, because of the low ion energies, no micromasking occurs on etching surfaces due to atomization and redeposition of mask material that has been vaporized or reacted to form a nonvolatile product, so that smooth etching surfaces without any unwanted structures are obtained. Nevertheless, there is little or no use for spontaneously reacting etchant species in plasma methods because they perform etching isotropically, i.e., they also etch under the edges of the mask, due to their spontaneous reaction properties. However, high anisotropy without underetching beneath the mask is advantageous for accurate transfer of structure.
A known possibility of combining the advantages of spontaneous and ion-induced plasma etching is to use side wall protection by a thin film deposited during the operation. Only a few ions strike the side wall of the structures to be etched, so it is coated by a film of inorganic or organic material, e.g., a polymer, from the plasma and is thus protected from attack by the spontaneously reacting etchant species. The substrate remains free of this protective film due to the simultaneous action of a high current of low-energy ions, and therefore it can be etched at a high rate. This yields anisotropic profiles and prevents underetching of the edges of the mask. However, when protective species, i.e., passivating species, are present in the plasma simultaneously with aggressive species, i.e., etchant species, this yields the disadvantage that strong recombination of the unsaturated passivation species with the spontaneously reacting etchant species eliminates these species in pairs as they form an inert reaction product, which greatly reduces the density of the available active species. To prevent this recombination of etchant species and passivation species, these species are separated from one another and used in alternation in a method according to the present invention. However, a new side wall area is exposed during each etching step and cannot be covered again and protected until the next passivation step, so a marked scoring of the side wall would normally result with this procedure. This effect is extremely undesirable and would make this method useless per se; according to this approach, this scoring could be avoided only by selecting a very small width and therefore a short duration of the individual etching step, but this in turn results in a very low etching speed or extremely frequent short-term alternations between etching and passivation, which would not be feasible because of the gas volumes in the supply lines and the plasma chamber, which also makes the latter method useless.